The study of DNA tumor viruses that transform rodent and human cells has led to a greater understanding of the molecular events that program the malignant state. In particular, investigation of the viral oncoproteins specified by the Simian Virus 40 Early Region (SV40 ER) has revealed host cell pathways, whose perturbation play an essential role in the transformation of mammalian cells. Over the last several years, we have focused on the role of the SV40 small t antigen (SV40ST) in human cell transformation. During the last funding period, we have confirmed that the interaction between SV40ST and PP2A contributes directly to human cell transformation. In addition, we have found that the perturbation of PP2A by SV40ST transforms human cells in a manner similar to that induced by cancer-associated mutations of PP2A subunits. These observations identify PP2A as a tumor suppressor in human cancers and suggest that a deeper understanding of the mechanisms by which SV40ST perturbs PP2A function will provide additional insights into tumor initiation and maintenance. In order to understand further the role(s) of PP2A in cell transformation, we propose to combine genetic, biochemical and cell biological approaches to characterize the PP2A complexes involved in cell transformation, to identify the role(s) of RalA in cancer development and to elucidate the molecular pathways perturbed by SV40ST and PP2A in human cell transformation. Investigating the regulation and function of PP2A in cancer development will not only enhance our mechanistic understanding of this tumor suppressor family but will also provide new insights into the pathways that help program the malignant state. In addition, these studies will provide a foundation for strategies to target these pathways therapeutically.